A radiation thermometer is for measuring heat radiation light intensity (heat radiation intensity) radiated from a measuring target object, so as to obtain the temperature of the measuring target object. This radiation thermometer has a characteristic of being able to measure the temperature of the measuring target by a relatively short time without contacting the measuring target object, and thus has a high industrial value. When the temperature of a measuring target object is measured particularly under the circumstances that conditions of temperature, pressure, atmosphere, and the like are largely different from the external environment, the aforementioned characteristic is exhibited effectively. Moreover, when the measuring target object moves, the characteristic of the radiation thermometer of being a non-contact type is important.
Examples of industrial application using the radiation thermometer include production of semiconductors, production of compound semiconductors containing a nitride system, and the like. In order to produce high-quality semiconductors with high purity, in most cases, the interior of a manufacturing apparatus is isolated from the outside and a substrate retained in the interior of the manufacturing apparatus is heated to a high temperature. In particular, metal organic chemical vapor deposition (MOCVD) for performing film formation on a substrate by subjecting to a chemically reactive material gas, or molecular beam epitaxy (MBE) for forming a film on a substrate by evaporating constituent elements of a semiconductor in a high vacuum is well known.
For these semiconductor manufacturing apparatuses (one example of a film forming apparatus), very precise temperature measurement is required so as to favorably keep uniformity and repeatability of semiconductors produced. As a specific example, the temperature of a measuring target object (film formation target object) is in the range of 500° C. to 1200° C., and measurement precision is ±2° C. or less. In practice, in production of a light-emitting element with a multiple quantum well constituted of InGaN (indium gallium nitride) and GaN (gallium nitride) being a light-emitting layer, in the process of producing this light-emitting layer, the substrate is retained at a certain temperature determined from other manufacturing conditions within the range of about 700° C. to 800° C. This certain temperature largely affects the emission wavelength of the light-emitting element, and thus precise temperature measurement as described above is necessary for realizing high emission wavelength uniformity and repeatability.
On the other hand, in order to accurately measure the temperature of a measuring target object by using the radiation thermometer, a value of emissivity of the measuring target object is necessary. As the temperature of an object increases, heat radiation light intensity from the object increases, and thus it is possible to measure the temperature of the object by measuring the heat radiation light intensity from the object. However, heat radiation light intensity from a general object is smaller than heat radiation light intensity from a blackbody at the same temperature. The emissivity is obtained by dividing heat radiation light intensity from an object at a certain temperature by the heat radiation light intensity from the blackbody at the same temperature. Therefore, by measuring the heat radiation light intensity from an object and dividing this heat radiation light intensity by the emissivity of this object, the heat radiation light intensity radiated by the blackbody at the same temperature as this object can be obtained, and the temperature of the object can be calculated from this heat radiation light intensity. The radiation thermometer using the principle described here can respond to changes of optical parts in various configurations from the radiation thermometer to the measuring target object by performing calibration at an appropriate temperature, without performing calibration using the blackbody in a wide temperature range.
The emissivity is measured with various materials, and is published in various documents. In general, many radiation thermometers have a function to store emissivity and use it to correct the heat radiation light intensity from an object, and when the emissivity of the material of a measuring target object is known by a document value or the like, this can be stored for use in the radiation thermometer. However, the emissivity depends not only on the material of the measuring target object but also on the surface condition and temperature. In this sense, the published emissivity is difficult to be used for precise thermometry.
On the other hand, under certain limited conditions, it is possible to measure emissivity. That is, in the wavelength range of light for measuring heat radiation light intensity, when the light does not pass through the measuring target object and the light irradiated to the surface of the measuring target object does not scatter, the emissivity (ε) is represented by an equation ε=1−R, where R is the reflectivity of light of the surface of the measuring target object. Therefore, in the wavelength range of light for measuring heat radiation light intensity, when the surface of the measuring target object has sufficient specularity and it is possible to measure the reflectivity of the measuring target object by using an external light source, and the measuring target object absorbs light, the emissivity can be obtained irrespective of the surface condition and temperature of the measuring target object. In order to measure the temperature of the measuring target object with high precision by using such a method, it is crucial to accurately obtain the heat radiation light intensity from the measuring target object and the reflectivity with a preset wavelength.
The radiation thermometer which measures the temperature while obtaining the emissivity of the measuring target object as described above is particularly important for substrate thermometry when a thin film is formed on the substrate. In the process of forming the thin film on the substrate, due to occurrence of interference of light by the thin film, the emissivity of the substrate including the thin film changes constantly as the film formation proceeds (as the film becomes thicker). By this change in emissivity, even when the temperature of the measuring target object is constant, the heat radiation light intensity from the measuring target object changes. Even in such case, when conditions as described above are satisfied, correction of emissivity can be made by measuring the reflectivity appropriately by using the external light source.
However, in the conventional radiation thermometer, heat radiation light intensity from a narrow region on a measuring target object is measured for precisely measuring the heat radiation light intensity in a film formation process. This is because when non-uniformity due to a measurement position exists in a film thickness distribution or the like of a thin film formed on a surface of the aforementioned measuring target object, there may be cases that the heat radiation light intensity or the reflectivity to be measured cannot be measured precisely due to the influence of this non-uniformity of the thin film. On the other hand, when the measurement region for measuring the heat radiation light intensity becomes narrow, the signal intensity of the heat radiation light becomes low and noise becomes large, thus causing a problem that a measurable lower limit temperature becomes high.
An object to be achieved by the embodiments of the present invention is to provide a film forming apparatus and a thermometry method capable of lowering the measurable lower limit temperature while suppressing decrease in thermometry precision.